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PIPING PHENOMENA ATOSTROVUL CORBULUI DIKE
Case Study
ing. Gaftoi Daniel Andrei
PhD student at TUCEB
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Presentation Content
I. Brief description of the site, of its geology and of the encountered phenomena
II. Considered assumptions causes of the piping phenomena
III. Confirmations of the considered assumptions using a mathematical model
IV. Proposal of corrective measures
V. Confirmation of the efficiency of the solutions using mathematical model
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Site description
Danube
TransformerStation
Pumping
Station Drainage
Channel
Ostrovul Corbului embankment dike :
part of the Iron Gates II hydropower plant
founded on highly permeable and easilytrained alluvial soils
initial project didnt supply a cutoff wall
the excessive water from the protected area is
pumped back into the storage lake
the pumping station is equipped with 2
high capacity pumps
it was placed on a caisson founded on the
bed rock at approximately 18 m depth
pumping water from the channel isintermittent ( 2- 3 rounds / day)
the duration of a pumping round 3 h
during which the water level lowers with
2 - 2.5 m
the water level difference between storage
lake and drainage channel is approx. 5 m
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Site geology
I silty sand layer (depth range : 3.3 m 10.2 m ) Un = 20 - 25
II, III coarse silt layer (depth range : 10.2 m 18.1 m ) Un = 50 - 150
icr = 0.18 0.30
Values of the permeability coefficient are high and very high k = 71 982 m/day
Size analysis for soil layers presented in the foundation of the dike
Hydraulicgradient(i)
Coefficient of uniformity ( Un)
Dangerous zonefor piping
Safe zone
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Aim of the study
to identify the potential causes of the piping phenomena
to identify the possible remedial solutions
Groundwater movement between the retention level and the lower levels created by waterpumping lead to soil structure damage - repeatedly ground subsidence near the
transformer station (observed since 2003)
Repeatedly filling of the hole withselected gravel no influence onstopping the piping phenomena
Affects the PS stability and
functionality
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Considered assumptions
Causes of the piping phenomena :
dam foundation soil that up to 18-20 m depth is composed of irregularsand and gravel, unstable in terms of hydrodynamic picking up
pumping station foundation which represents an obstacle in the flownet of the groundwater flow from the dam to the drainage channel.
no correlation between the pumped flow and the water level from thechannel
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Mathematical model 2D horizontal FEM model
Pumping
stationarea
Danube
Meshed region includes a large zone 2707 elements
The results are not influenced by domain limit boundary conditions
Finer mesh in the pumping station area better results in the interested area
T = 1200 m2/day k = 0.0016 m/sec
saturated volume water content - 20%
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Verify the assumption
Initial boundary conditions :
H = 42.00 maSL in storage lake
H = 37.00 maSL in thedrainage channel
Situation modeled a pumping round (first step before the pumping round)
Value of hydraulic gradient:
i = 0.18 0.20 i icr
lines of constant head
hydraulic gradients and flow vectors
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Verify the assumption
Boundary conditions :
H = 42.00 maSL in storage lake
H = 35.00 maSL in thedrainage channel
Situation modeled a pumping round (last step after the pumping round)
Value of hydraulic gradient:
i = 0.26 i > icr
Geometry of the drainage channel
Cause of the hydraulic gradientsconcentrations
lines of constant head
hydraulic gradients and f low vectors
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Corrective measures proposed
1. Changes to the pumping rate - limit the water level offset into thechannel and the lowering speed of the water (1 m in 3 hours)
2. A sheet pile cutoff wall - create a hydraulic barrier to reduce the flowgradients below the critical values
3. The combined effect of the two changes to the pumping rate and asheet pile cutoff wall
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Corrective measures
Boundary conditions :
H = 42.00 maSL in storage lake
H = 36.00 maSL in the drainagechannel
Situation modeled changes to the pumping rate 1 m in 3 hours
Value of hydraulic gradient:
i = 0.22 i > icr
lines of constant head
hydraulic gradients and f low vectors
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Corrective measures
Boundary conditions :
H = 42.00 maSL in storage lake
H = 35.00 maSL in thedrainage channel
Value of hydraulic gradient:
i = 0.15
Situation modeled sheet pile cutoff wall
lines of constant head
hydraulic gradients and flow vectors
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Corrective measures
Boundary conditions :
H = 42.00 maSL in storage lake
H = 36.00 maSL in thedrainage channel
Value of hydraulic gradient:
i = 0.1
Safety factor 1.8
Situation modeled sheet pile cutoff wall and changes to the pumping rate
lines of constant head
hydraulic gradients and flow vectors
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Conclusions
The paper aims to identify the causes of the subsidence phenomenaobserved at Ostrovul Corbului pumping station and to define theremedial solution.
The analysis on the mathematical model showed that hydrodynamic
picking up of the sand from the dam foundation has 4 principalcauses:
the configuration of the terminal area of the drainage channel the presence of the caisson the presence of easily trained alluvial soils the pumping regime - 2.00 2.50 m in 3 hours
To control the seepage phenomena we proposed :
the development of a sheet pile cutoff wall
changes to the pumping rate
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Thank you for your attention !
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